Hydraulic lash adjusters for internal combustion engines have been in use for many years. Hydraulic lash adjusters are used to eliminate clearance or lash between engine valve train components which occur under varying operating conditions, in order to maintain engine efficiency and to reduce noise and wear in the valve train.
Hydraulic lash adjusters operate by transmitting the energy of the valve actuating cam through hydraulic fluid trapped in a pressure chamber beneath a plunger. During each operation of the cam, as the length of the valve actuating components vary due to temperature changes, small quantities of hydraulic fluid are permitted to enter or escape from the pressure chamber. As the hydraulic fluid enters or escapes the pressure chamber, the position of the plunger is adjusted and consequently the effective total length of the valve train is adjusted which minimizes or eliminates the lash.
In current hydraulic lash adjusters, the escape of hydraulic fluid from the pressure chamber is most commonly accomplished by a leak path located between the plunger and the wall of the lash adjuster body member. Such escape or "leakdown" through these leak paths is controlled solely by the fit of the plunger within the body. The manufacture of these leak paths requires close manufacturing tolerances between the plunger and the body member, which is typically an expensive operation. Such prior art lash adjusters are disclosed, for example, in U.S. Pat. Nos. 4,438,739, 4,481,913, 4,462,364, 4,633,827, and 4,840,153.
Another system for minimizing lash is shown in U.S. Pat. No. 5,622,147. This configuration eliminates leak paths between the plunger and the cylinder and instead uses a normally open ball check valve, with limited travel, in place of a normally closed ball check valve.
None of these prior lash adjusting systems adequately compensate for the rapid growth of the exhaust valve stem which can occur immediately after a cold start and can cause damage to the valve or valve seat. This is partly because while hydraulic lash adjusters can increase their length quickly, they require more time to contract. The shrink rate of the lash adjuster is a function of the oil viscosity and temperature. When the oil is cold, the shrink rate is slow because the leak path between the plunger and the lash adjuster body remains constant.
From a cold start, current lash adjusters have difficulty compensating for the initial valve growth rate during approximately the first 2,000 engine cycles. This can result in operation problems. Increase growth can result in the exhaust valve hanging open, which in turn can result in the exhaust valve becoming even hotter and growing even more. This can further result in loss of power output and deposit buildup on the valve stem.